1. Field of the Invention
The present invention relates to a photosensitive resin composition for forming solder resist films, etc., a cured coating film thereof, and a circuit substrate formed of such a cured film.
2. Prior Art
A circuit substrate, for instance, a printed circuit substrate is fabricated by forming a circuit interconnecting pattern on a copper-clad laminate, for example. One circuit unit can then be formed by mounting electronic components on the circuit substrate. The electronic components such as capacitors and resistors are connected and fixed by soldering to a copper foil land, i.e., a soldering land of the circuit interconnecting pattern.
An insulating film called a solder resist film is provided over all surface portions of a circuit interconnection-including substrate except a soldering land, thereby ensuring that any possible soldering of interconnections that must not be connected together is avoided and any possible surface corrosion of copper is prevented.
The soldering of electronic components on a printed circuit substrate is achieved by a so-called flow soldering process wherein a chip form of electronic component having electrodes at both its ends is provisionally held at a given position of the printed circuit substrate such that the electrodes are located at a soldering land, and the provisionally held electronic component is allowed to come in contact with a flow of molten solder, so that the electronic component can be soldered on the soldering land, or a so-called reflow soldering process wherein while the electrodes of an electronic component are located at a soldering land precoated with solder paste, as mentioned just above, solder powders of the soldering paste are molten by heating, so that the electronic component can be soldered on the soldering land.
Regardless of whether the flow or the reflow soldering process is used, the electronic component and soldering land are allowed to come in contact with a flow of molten solder or are coated with solder paste after a flux has been applied on the soldering land. By doing so, the oxidation of copper foils of the soldering land due to heat, etc., generated during soldering can be avoided to make soldering satisfactory. The soldering land of a printed circuit substrate is heated to 200.degree. C. to 200.degree. C. during soldering. If the soldering flux is applied on the soldering land to form flux coating thereon, then the flux film cuts off oxygen to prevent oxidation of the copper foils and reduces the oxides that have already occurred, so that the molten solder can be well wettted. This is true even when the surface of the soldering land is exposed or provided with a protecting film.
For such a flux, a rosin type of resin is often used. This flux is composed mainly of rosin and contains activators such as amine halides and organic acids as subordinate components, all dissolved in an alcoholic solvent. When an ordinarily used flux having a solid content as high as 10% is used with any desired coating techniques such as dipping and foaming, a coating thickness of at least a few micrometers is achievable. Thus, even when the surface to be coated has relatively large asperities, it can be well covered with a uniform coating. However, the coating film contains ionic substances, and the rosin used has a softening point of about 100.degree. C. or is soft, so that dust is likely to stick thereto. To make better insulation between soldering lands, the coating film must be washed and, to this end, it is required to use an organic solvent.
In recent years, however, diffusion of a fluorine type of solvents in the air has caused grave concern in view of environmental pollution, and so the use of such solvents, especially flon is, or will be, prohibited. To cope with this, a so-called washing-free yet low-residue type of flux is now under development so as to fabricate products without recourse to any washing step responsible for diffusion of an organic solvent while a flux film remain intact thereon. The flux used must have a solid content as low as 2%. However, such a flux film is thin; so it is remarkably affected by the physical properties of the surface to be coated. Even when the application surface is a solder resist film, its physical properties often cause the flux film to be irregularly coated.
For instance, when soldering is carried out with a flow soldering apparatus, so-called solder balls of several tens micrometers in diameter are often left on a flux film. The solder balls are once oxidized, but are then reduced by the reducing agent contained in the flux. Since the force with which the solder balls stick to a flow of molten solder is larger than that with which they stick to the flux film or portions of the flux film sticking to the solder balls stick to the solder resist film, the solder balls are absorbed in the flow of molten solder, so that no solder can be left on the flux film or the solder resist film. Exposed surface portions of the solder resist film on which no flux film has been formed are intrinsically not readily wetted with molten solder. The solder balls, if occur, are oxidized by those surface portions to form oxides. Since the adhesive force of the oxides to the solder resist film is larger than the force with which they stick to the flow of molten solder, the oxides are hardly absorbed in the flow of molten solder; so they are likely to remain.
Thus, the solder balls tend to remain on the exposed surface portions of the solder resist film on which no flux film has been formed. The solder resist film used is usually formed of ultraviolet-curable resin or thermosetting resin. The solder resist film is now increasingly required to have high resolution and precision, because electronic components are mounted on a printed substrate at high density to make electronic equipment compact and, to achieve this, the interconnecting density of the printed circuit substrate must be increased with an increase in the interconnecting density between soldering lands. To this end, it has been proposed to use a liquid photo-solder resist composition. For instance, JP-A-50-144431 and JP-A-51-40451 disclose a process for forming a solder resist film by coating a photosensitive composition comprising a bisphenol epoxy acrylate, a photosensitizer, a thermosetting epoxy compound with a setting agent therefor, and the like all over a printed circuit substrate by screen printing, removing the solvent from the coating film by volatilization, exposing the coating film to light while a negative film with an interconnecting pattern formed thereon is placed on the coating film, and developing the exposed coating film with a solvent to dissolve and remove an unexposed non-image portion therefrom.
Thus, a solder resist film is developed using a photosensitive composition developable with a dilute alkali solution in order to eliminate the risk of environmental pollution and fires which may otherwise be caused by the use of a photosensitive composition in which such an organic solvent is used for developers or which itself contains an organic solvent, and of an organic solvent type of developer. However, when such a solder resist film, especially one having a glossy appearance is used with a washing-free yet low-residue type of flux, non-uniform coating tends to occur, that is to say, local portions of the solder resist film are not coated at all.
In the case of products having a solder resist film with solder balls remaining thereon, the solder balls move about on the solder resist film while the products are handled. Especially in the case of a recently developed printed circuit substrate having an interconnecting density of about 100 .mu.m, the interconnections are often short-circuited. To avoid this, it is required to remove the solder balls. However, it is not easy to remove solder balls from all interconnections for as many as several thousand components mounted on one printed circuit. Thus, problems arise in connection with productivity as well as reliability.
Among conventional photosensitive resins of the type that is developable with an aqueous solution of dilute alkali, for instance, there are a photo-solder resist composition comprising a reaction product obtained by the reaction of a reaction product between a novolac epoxy resin and an unsaturated monocarboxylic acid with a polybasic acid anhydride, a photopolymerization initiator, a diluent and an epoxy compound (see JP-B-1-54390), a one-pack type of photo-solder resist comprising an adduct of a portion of carboxyl groups of a (meth)acrylic ester-(meth)acrylic acid copolymer with a (meth)acrylic ester having a terminal epoxy group, a diluent and a photopolymerization initiator (see Japanese Patent Application No. 1-311710), and a photosensitive composition comprising a novolac resin and an onium compound (see JP-A-60-175045 and JP-A-3-154059). These compositions are developable with an aqueous dilute alkali solution due to the presence of carboxyl groups introduced therein, and an onium type of component used therewith. However, an organic solvent is still needed when they are coated on a printed circuit substrate, and care must taken of post-development disposal of the alkali solution used as well. Thus, the aforesaid compositions have yet to be improved in view of mass fabrication. To achieve this, an alternative photosensitive resin composition which is not only developable by use of water alone but is also capable of forming a high-precision pattern with sensitivity improvement is put forward (see JP-A-6-59448). In this case, too, solder balls remain unavoidably due to such non-uniform coating as mentioned above. An improvement in this regard is desired.
As explained above, parts and devices are mounted on a printed circuit substrate at high density and with high precision by soldering while such parts and devices are provisionally held in place. To meet recent demands with increasing severity, care should be taken of the following requirements at much higher levels. Any possible detachment of the parts, etc., must be reduced as much as possible by enhancing the adhesive force of the adhesive used. When many parts are soldered on a printed circuit substrate, the number of points at which no predetermined soldering occurs is counted with an optical, three-dimensional automatic appearance inspector designed to pass judgment on whether soldered joints are acceptable or not. In this case, false data--actually acceptable soldered joints are often judged as unacceptable due to the reflection of light from the surrounding glossy solder resist film--should be reduced. Judgment is passed on whether the operation of a given circuit is acceptable or not with an inspector. In this case, the contact of an ICT pin of the inspector (a pin of an in-circuit tester that comes in contact with the circuit) with the circuit must be in good-enough condition. Improvements in these points are also desired.
A first object of the present invention is to provide a photosensitive resin composition capable of forming a cured film such as a solder resist film on which a liquid material such as a washing-free yet low-residue type of flux can be uniformly coated, a cured film thereof, and a circuit substrate fabricated using such a cured film.
A second object of the present invention is to provide a photosensitive resin composition capable of forming a cured film such as a solder resist film which is unlikely to produce molten metal balls such as solder balls during soldering, even when it is used with a coating composition having a low solid content such as a washing-free yet low-residue type of flux, a cured film thereof, and a circuit substrate fabricated using such a cured film.
A third object of the present invention is to provide a photosensitive resin composition capable of forming a solder resist film or the like with high resolution and high precision, a cured film thereof, and a circuit substrate fabricated using such a cured film.
A fourth object of the present invention is to provide a photosensitive resin material capable of forming a solder resist film as an example, which is improved in terms of heat resistance, adhesion to an application surface, chemical resistance, solvent resistance, electrical properties, and mechanical properties, a cured film thereof, and a circuit substrate fabricated using such a cured film.
A fifth object of the present invention is to provide a photosensitive resin composition capable of being developed with a dilute alkali solution to form a solder resist film or the like, a cured film thereof, and a circuit substrate fabricated using such a cured film.
The sixth object of the present invention is to provide a photosensitive resin composition capable of being developed by use of water alone to form a solder resist film or the like, a cured film thereof, and a circuit substrate fabricated using such a cured film.
A seventh object of the present invention is to provide a photosensitive resin composition capable of providing a cured film such as solder resist film which makes it possible to enhance the adhesion strength of the adhesive used for provisional fixation of parts and devices such as surface mount devices and strap lines, reduce false data output of a three-dimensional automatic appearance inspector, and improve the contact of an ICT pin with a given circuit, a cured film thereof, and a circuit substrate fabricated using such a cured film.
A eighth object of the present invention is to provide a photosensitive resin composition which substantially eliminates the risk of environmental pollution and fires, a cured film thereof, and a circuit substrate fabricated using such a cured film.
A ninth object of the present invention is to provide a circuit substrate with no electronic components mounted thereon, which is improved in terms of soldering productivity and reliability.
An tenth object of the present invention is to provide a circuit substrate with electronic components mounted thereon, which is improved in terms of productivity without detriment to circuit performance, and is inexpensive as well.